The present invention relates to polyol blends that are capable of reacting with isocyanate compounds to form polyurethane foams having a high degree of compressive strength and shock attenuation over a wide range of static loading such that the foams are useful in a wide variety of packaging applications.
Polyurethane foams are currently used, inter alia, to package articles as a means for protecting such articles as they are being shipped and handled. The foams may be pre-formed into molded shapes that correspond to the shape of the packaged article. Often, two pre-formed foam pieces are used, with one of the pieces being placed under the article while the other piece is placed above the article. The resultant foam-article-foam `sandwich` is then placed in a shipping carton, whereby the two pieces support and encapsulate the article during shipping and handling.
Alternatively, polyurethane foam may molded "in place," i.e., about the article, essentially using the article as a forming mold. This is generally accomplished by mixing the necessary reactants to make polyurethane foam (generally a polyol, an isocyanate compound, and other materials as discussed below) in a vented bag, placing the bag in the bottom of a shipping carton, placing the article to be packaged atop the bag as the reactants begin to expand into a foam, and then placing a similar bag with an expanding polyurethane foam atop the article. If desired, the foam reactants may simply be mixed in the bottom of the carton, i.e., without a bag, with a sheet of film placed atop the expanding foam and with the article placed atop the sheet. A second sheet of film is then placed over the article and additional foam is formed thereon. In either case, the carton is finally closed to complete the process so that, as with the pre-formed foam package, the two molded-in-place foam pieces support and encapsulate the article for shipping and handling.
Polyurethane foams for both pre-formed and in-place packaging are typically produced by the reaction of an isocyanate compound with a hydroxyl-containing material, such as a polyol (i.e., a compound that contains multiple hydroxyl groups). The most common isocyanates are toluene diisocyanate (TDI) and methylene diphenylisocyanate (MDI), particularly the latter. As with any foaming process, a blowing agent must be used to expand the resultant polymeric structure into a cellular structure. Traditional blowing agents that have been used include fluorocarbons, chlorofluorocarbons, and other halogenated hydrocarbons. However, such blowing agents are not preferred for environmental reasons and cost. Instead, it is preferred to react the isocyanate and polyol in the presence of water. The water reacts with the isocyanate compound to produce carbon dioxide which, in turn, serves as the blowing agent by causing the polyurethane to expand into a foam.
Cellular polyurethane foams are typically classified as being either rigid or flexible. Rigid polyurethane packaging foams are generally closed-cell foams characterized as having a relatively high degree of compressive strength, e.g., typically greater than about 14 psi.
As used herein, the term "compressive strength" refers to a numerical physical property value of a foam that is determined from a point on a stress v.s. deformation (i.e., deflection) curve for that foam at the yield point or at 10% deformation, whichever point occurs first as compressive stress increases, as measured in accordance with ASTM D 1621. Externally applied stress deforms the cell structure of foams. For foams that exhibit a sudden collapse (failure) of the cells at a certain level of applied stress, the value at the maximum deformation point of the curve (i.e., just prior to failure) corresponds to the compressive strength of the foam at the yield point. For foams that do not exhibit a definite failure point, the value at 10% deformation is used to represent the compressive strength of the foam. The compressive strength is expressed in terms of stress/unit area of the foam at which stress is applied.
Rigid polyurethane foams are often employed in packaging applications in which the packaging foam will be subjected to high static loading, e.g., of 1 psi and above, such as is encountered when packaging heavy articles. Examples of such articles include industrial machinery, electric motors, engines, transmissions, synthetic stones, etc. Because of their high compressive strength, rigid polyurethane foams have traditionally been used in such applications due to their ability to support heavy articles both statically and when the foam is subjected to high compressive forces, e.g., when the package is jostled during shipping and handling.
A major drawback of rigid or polyurethane foams, however, is that their ability to absorb and attenuate impact shocks and vibrations is often insufficient to properly protect the packaged article. That is, while rigid polyurethane foams function well in supporting and restricting the movement of packaged articles, such foams often transmit external shocks and vibrations to the packaged article in amounts that exceed the maximum amounts that the packaged article can withstand without suffering damage. The susceptibility of articles to shock or vibration damage, known as the "fragility" of the article, is conventionally expressed in terms of a "G" value. That is, "fragility" refers to the maximum shock that a packaged article can withstand without suffering damage, wherein such maximum shock is measured as the number of Gs, the gravitational constant, transmitted to the article. The more susceptible an article is to damage, the lower the number of Gs that can be transmitted to that article without damaging the article. Thus, "very delicate" articles (such as aircraft altimeters) may have a fragility of about 15 to 40 Gs; "delicate" articles (such as computer disk drives) may have a fragility of about 40 to 80 Gs; "moderately rugged" articles (such as TVs and VCRs) may have a fragility of 80 to 100 Gs; and "rugged" articles (such as furniture) may have a fragility of about greater than 115 Gs. Due to their low attenuation of transmitted shock, rigid polyurethane foams are generally useful for packaging articles having a fragility of 115 Gs and higher.
When packaging more fragile articles having lower G values, i.e., less than about 80 Gs, flexible polyurethane foams are generally employed. This is because, in comparison to more rigid polyurethane foams, flexible polyurethane foams absorb and attenuate external shock and vibration to a higher degree so that a smaller proportion of the shock or vibration is transmitted to the packaged article. In this manner, delicate and very delicate articles are less likely to be damaged when packaged in flexible foams than when packaged in rigid foams.
In general, however, flexible polyurethane foams, which are generally open-cell foams, have a lower compressive strength and less load bearing capability than their more rigid counterparts. This necessitates either packaging only relatively lightweight articles in flexible foam or using a sufficient amount, i.e., thickness, of the flexible foam to compensate for the load bearing and compressive strength qualities of the foam. The former option is undesirable in that many articles requiring relatively high load-bearing and compressive strength capabilities also have low G values, while the latter option is undesirable because it adds extra cost to the package.
Accordingly, a need exists in the art for a polyurethane foam having a relatively high degree of compressive strength and with excellent shock and vibration absorbing characteristics over a broad range of static loading conditions so that a wide variety of articles, including those having fragility values below about 40-45 Gs, can be packaged by the foam with minimal foam thickness being required.